Radio Frequency (RF) signals and components are employed in a variety of devices, including mobile communication devices such as mobile telephones. One type of commonly employed RF component is an RF switch, which is sometimes employed to selectively connect an RF signal from an input terminal to an output terminal. For example, an RF switch might be employed to selectively connect an antenna of a wireless mobile device (e.g., a mobile telephone) with either transmit or receive circuitry of the device. In another application, an RF switch may be employed for selectively connecting a signal path among a plurality of band-switching filters.
Some RF switches utilize a field effect transistor (FET) as the switching element. When operated as a switch, drain and source designations are typically not meaningful as the FET is operated in a symmetric fashion.
In general, there are a number of parasitic capacitances associated with a FET switch arising from the FET itself, and from various resistive and capacitive components employed in the switch, and from electrical traces connecting these components. These capacitances can degrade various aspect of the FET switch performance. In particular, parasitic gate capacitance to ground degrades the linearity of the FET switch and results in degraded harmonic and intermodulation performance for the FET switch.
In the past, a typical way to reduce these parasitic capacitances has been to make the wafer or substrate on which the FET is provided so as to decrease the parallel plate capacitances. However, this approach can only be taken so far before the interconnections to a mating board for the switch become excessively long, causing performance problems.
What is needed, therefore, is a FET switch for RF signals with an architecture that can produce reduced stray or parasitic capacitance values from the gate to ground.